Historically, various foodstuffs such as sausages and cheeses have been produced in casings that were either natural, for example, as derived from animal intestines, or synthetic, such as those made from collagen or cellulose. The foodstuff is introduced into the casing and because of the permeability of these types of casing, the foodstuff, such as sausage meat, can be exposed to wood smoke, available in both gaseous and liquid forms, which imparts a flavor and/or color component to the sausage.
Currently, sausage manufacturers are interested in having another type of synthetic casing based on polymers and, in particular, thermoplastics available. These thermoplastic casings need to be porous enough to be smokable by liquid and/or gaseous smoke. Although plastic casings are well-known in sausage manufacturing, smokable plastics pose unique problems because of their impermeability to air and liquids.
Currently, thermoplastic casings are primarily larger sized casings, and because of their oxygen barrier properties, may remain on the meat product after processing. However, there is a demand for smaller diameter casings to produce hot dog-type sausages, and generally these types of casings must be removed from the processed sausage, although some manufacturers may prefer to leave the casing on as an oxygen barrier when sold to the consumer, who will remove it prior to heating and eating.
Some thermoplastic casings are made from polyamides, either monolayer or multilayer films having at least one polyamide (nylon) layer. Casings comprising a blend of nylon with polyester are also known, as are multilayer casings of nylon with ethylene polymers or copolymers. These types of casings also may have a moisture content ranging from about 1-3 weight % (wt. %). Multilayer casings, such as those having an outer layer of nylon and an inner or food contact layer of an absorbent material such as a cellulosic film or a polymer with an open cell structure, are also known. There are thermoplastic casings that are smokable with gaseous smoke and that produce a range of colors and flavors, from slightly reddish to that approximating the color seen when cellulosic casing is used and smoked.
Smoke is available in both liquid and gaseous forms. Both are well-known to those in the sausage and food processing industry. The liquid version is often used in processing operations where linked sausages are sprayed or pulled through baths of the liquid smoke, generally at room temperatures. The moisture barrier properties of thermoplastic casings generally impede the transfer of liquid smoke through the casing to the enclosed meat. Cellulosic casings are preferred for color and taste transfer using liquid smoke, as well as with gaseous smoke.
What is still needed are plastic casings that may be produced at high speeds, that will be stiff enough to maintain their shape once stuffed with the foodstuff, that do not wrinkle during sausage production, and that are porous enough to allow smoke components present in both liquid smoke and gaseous smoke to pass through to the encased food.
As discussed in U.S. Pat. No. 4,303,711, “Tubular film for packing and casing paste-type foodstuffs especially sausage and cheese,” Erk et al., issued Dec. 1, 1981, “single-layer, unstretched plastic casings consisting of higher homo-polyamides (polyamide 11 and polyamide 12)” are known, as well as plastic casings consisting of such polyamides coextruded in two layers with polyamide 6 as the outer layer. These polyamide casings may be easily made by the blown film technique, but typically suffer from a lack of dimensional stability and uniformity, being deformed upon stuffing so that production of stuffed sausages to a uniform diameter is difficult. The '711 patent further indicates that these unstretched films suffer from an undesirably wrinkled appearance after cooking and chilling. These casings are meant to be left on the finished sausage and, therefore, a wrinkled appearance is a negative trait. Additionally, this casing is not meant to be porous, but rather to be impermeable to moisture and oxygen.
The contents of each patent and publication identified herein are incorporated herein to this application in their entirety by this reference.
To overcome the defects or inadequate performance of seamless thermoplastic casings produced by blown film technology, plastic casings have been produced using stretch orientation.
Also, various attempts have been made at making stretch oriented polyamide casings. Uniaxially stretched casings, which are only stretched in the longitudinal (“machine”) direction (“MD”), reportedly have the same disadvantages as unstretched casings regarding insufficient dimensional stability, nonuniformity of diameter, and excessive wrinkling.
U.S. Pat. No. 4,560,520, “Single-layer elastic tubular film of polyamide used for packaging paste substances and a process for the production of such film,” Erk et al., issued Dec. 24, 1985, discloses forming multiaxially stretched, monolayer, polyamide, tubular films, e.g., of nylon 6 or nylon 66, which have elastic properties and which are to be used for packaging table sausages and boiled sausages. The disclosed films are “fixed thermally” and shrunk after stretching, e.g., by subjecting the tube to controlled shrinkage of at least 15% and at most 40% at temperatures above 90° C. and also subjecting the film to infrared irradiation. This is to produce a nylon casing that does not have shrinkage at temperatures under 90° C. This pre-shrunk casing is used for stuffing with meat emulsion and relies upon its elastic properties to provide wrinkle resistance. Again, this casing is meant to be left on the finished sausage, and to protect it from air and water while so encased. It is not permeable to any coloring or flavoring agents used on the outside of the encased sausage.
Additionally, orientation of seamless tubes of nylon by biaxial stretching is difficult. Extrusion and orientation of multilayer tubes, especially coextruded tubes, containing mixed layers of polyamides and other materials having different melting points, melt viscosities, and a different affinity for water can be very difficult. For example, U.S. Pat. No. 4,892,765, “Heat-shrinkable laminate film and process to produce the same,” Hisazumi et al., issued Jan. 9, 1990, notes that although it is desirable to extrude films for packaging hams and sausages in tubular form, it is difficult to make a stretched tubular polyamide film of uniform thickness. Hisazumi et al. disclose production of a heat-shrinkable multilayer film having a core layer of a polyvinylidene chloride copolymer attached to opposing polyamide layers (e.g., of nylon 6/66 copolymer) by opposing adhesive layers. This film is made using an orientation process that utilizes water to soften and plasticize the nylon to a degree sufficient to allow or facilitate orientation.
Generally, attempts at biaxially stretching nylon multilayer casings have used polyamide as the outer layer of the tube. The orientation processes employed for nylon multilayer casings have tended to involve complicated apparatus and processing such as that found in U.S. Pat. No. 4,886,634, “Process and apparatus for the production of an extruded, stretched, seamless tube,” Strutzel et al., issued Dec. 12, 1989.
This type of casing or film is not meant to be permeable to air or water and, therefore, is of no use when producing sausages or food products treated with coloring or flavoring agents during the processing cycle.
A number of other thermoplastic casings that have a degree of permeability have been described in the literature.
In EP 0139888, “Process for smoking in thermoplastic casings foodstuffs that are either heated after packing or packed in a hot fluid state,” Erk et al., published May 8, 1985, linear aliphatic polyamides combined with other polymers such as an ionomer resin, modified ethylene vinyl acetate copolymers and/or modified polyolefins are taught for making smokable casings. Although under wet smoking conditions taste and fragrance is transferred, these casings have low gas permeability and it is believed that they result in products that are more moist than generally acceptable, and cannot replace cellulose or other natural casings.
Other high barrier casings in respect to oxygen and vapor have been described. For example, coextrusion of polyvinylidene chloride copolymers (“PVDC”) with polyamides or polyolefins have proven difficult because PVDC is very temperature and shear sensitive during extrusion. These copolymers are extrudable only over a narrow temperature range without causing degradation of the polymer in the extruder or die, which causes imperfections in the film. Polyamides require much higher temperatures for extrusion—generally about 200° C. or higher. At these higher extruder and die temperatures, coextrusion of polyamide with PVDC is difficult and degradation of the PVDC may be expected, even if the PVDC layer is insulated from polyamide layers by intermediate layers. Resulting imperfections in the film may detrimentally affect the appearance, strength, and barrier properties of the film and/or ease of orientation or biaxial stretching. In U.S. Pat. No. 5,084,310, “Smoked food-package and smoking process,” Hisazumi et al., issued Jan. 25, 1992, a mixture of PVDC and a low melting point polyamide is taught, producing a smokable polyamide. The resultant product allows the passage of a small amount of water vapor, but the smoked taste of the final product is much less than that of sausages made with conventional smoked cellulose casings.
U.S. Pat. No. 7,361,392, to Borodaev et al., titled “Packaging from a polymer film for food products,” issued Apr. 22, 2008, teaches a polymer film that is primarily polyamide with a hydrophilic compound, that forms a highly dispersed phase and is capable of mixing with at least 10 wt. % of water. Among the examples, two films are produced using two different polyvinyl alcohols as the hydrophilic component. One polyvinyl alcohol produced an acceptable film (i.e., smokable, good mechanical properties) while the other did not.
U.S. Pat. No. 4,851,245, “Smokable synthetic food-packaging film,” Hisazumi et al., issued Jul. 25, 1989, teaches a smokable food-packaging film comprising at least one layer of a mixture of polyamide and alpha-olefin vinyl alcohol copolymer. This film is claimed to have both gaseous permeability and oxygen barrier properties.
U.S. Patent Application Publication 2005/0163948 A1, “Smokable Polymeric Casing,” Owen J. McGarel, published Jul. 28, 2005, describes a thermoplastic casing made with a novel polyvinyl alcohol/nylon blend, which, when exposed to gaseous smoke, produced some color on the encased sausages.
In summary, although several of the aforementioned plastic casing products have gained varying degrees of commercial acceptance in different market segments, their advantage compared to the traditional cellulosic casing has been chiefly one of cost, but having a number of problems including those of dimensional stability, uniformity of diameter, lack of permeability to smoke or only of a low to moderate level of permeability to gaseous smoke and not liquid smoke, and wrinkling.